In general, there are three types of digital optical disks: read-only, recordable (also called Write-Once or Write-Once-Read-Many (WORM)) and erasable (also called rewritable). Examples of commercially available read-only optical disk technologies are the Compact Disk (CD) for digital audio and the Compact Disk--Read Only Memory (CD-ROM) for computer data. Compact Disk--Recordable (CD-R) drives and media are also commercially available. An example of an erasable (rewritable) optical disk technology is the Magneto-Optic disk, widely used for computer data storage.
For any electromechanical disk technology, whether magnetic or optical, there are physical limitations on information density. Some of these limitations are mechanical, for example, the tolerance on angular velocity for a disk rotation control system, or the tolerance on positioning a transducer relative to a data track. Other limitations are imposed by transducer or media physics, for example, diffraction effects in optics. In some technologies, for example many computer magnetic disks, a disk remains in a single drive for writing and reading. If the same drive writes and reads the disk, some repeatable mechanical effects may be ignored. For example, if the angular velocity of the disk is at the high extreme of an allowable range, this is not a problem since a disk recorded at the high speed will be read at the same high speed, so that data writing rates and data reading rates are identical. If a disk must be interchanged (written in one drive and read in a different drive), then the various worse case limitations of both drives combine, and the tolerances for each drive must be narrowed in order to meet an overall specification with interchange. For mass-produced read-only media, for example CD-ROM's, the process for writing a metalized master disk for molding of copies is typically a very high precision (and expensive) process. Very little of the overall allowable tolerances are used by the writing process, enabling most of the overall allowable tolerances to be used in the reading drives. Therefore, for example, portable read-only CD-ROM drives for notebook computers can have substantially broader tolerances (and substantially lower cost) than the drives producing the master disks for molding. However, if each of the drives involved in interchange can write data to a disk, then each of the drives must be limited to no more than half of any particular overall tolerance specification. For example, if the angular velocity at any particular radial head position must be accurate to .+-.1.0%, the angular velocity for each drive involved in interchange must be accurate to .+-.0.5%.
Some data formats assume that an entire medium will be recorded at one time. Other data formats assume that individual random sectors of data can be erased and rewritten. Typically, format provisions for rewriting individual sectors reduce the overall effective data capacity. CD, CD-ROM, and CD-R formats are designed for maximum data capacity with no provision for rewriting individual sectors. CD-ROM's are organized into data sectors and each data sector must be phase synchronized with the preceding sector and with the following sector. As a result, compatible CD-R disks must be written in sequence, starting from the first sector and writing each sector in the physical order that they appear on the disk. The standard CD format specifications do not support the ability to write or overwrite individual sectors with random access or to append to a partially recorded medium. Typically, recordable drives that can append to a partially recorded medium, and drives that can erase and overwrite previously recorded data, must provide data gaps for accommodating angular speed variations between drives and must provide additional clock synchronization patterns for accommodating clock differences between drives. For example, magnetic disks and magneto-optic disks are typically formatted into sectors, with each sector including a preamble for synchronizing a write clock, and with each sector including extra space at the end to allow for variations in angular velocity, each of which reduces effective data capacity. CD, CD-ROM, and CD-R formats do not have sector preambles for synchronization or extra space at the ends of sectors. In general, drives that can append or rewrite individual sectors with random access have a reduced effective disk capacity relative to drives, with the same bit density, that write an entire disk at one time.
In addition to clocking precision requirements and angular velocity precision requirements, writing drives must meet precision requirements for radial position or track following. Writable and rewritable optical disk media often have a predefined track, typically a land and groove structure. Other approaches to predefined tracks may be found in U.S. Pat. Nos. 5,213,859 and 5,204,852. Typically, for drives using grooves or similar approaches, the bandwidth and signal to noise ratio for track centering of the writing laser are not as good as that obtained by the high precision drives used for mastering read-only media. In addition, some servo approaches proposed for writable media may be incompatible with read-only formats.
Various digital optical disk standards are being planned in advance of available technology. That is, various capacities and formats have been proposed for future standardization, even though corresponding drives and media may not yet be available or practical. An example is erasable (rewritable) Digital Versatile Disks (DVD) (previously called Digital Video Disks). The proposed standards for erasable DVD's assume that the model established by CD's will continue. That is, the proposed standards assume that at any particular bit density, read-only and recordable (write-once) media will have the highest possible data capacity and erasable (rewritable) media will have a reduced effective data capacity. The proposed standards assume that erasable (rewritable) systems must have a lower capacity than read-only and recordable (write-once) systems due to the extra overhead for synchronization and gaps for drive speed variation. The proposed standards assume that writable media must have a land and groove structure or other predefined track servo information. In general, format differences between proposed erasable disks and read-only disks are incompatible, so that drives must be designed to read two separate formats, or drives designed only for read-only and write-once disks will not be able to read erasable disks. The proposed evolution or "migration path" typically specifies that for each new step in bit density, there will first be read-only products that extract the maximum possible capacity from the anticipated technology (shorter wavelength lasers and improved media characteristics), followed by recordable (write-once) products having the same capacity as the read-only products, followed by erasable products with the same bit density as read-only and recordable products, but with a lower effective capacity.
There is a need for erasable (rewritable) optical disks having the same format and the same effective capacity as read-only and write-once disks.